A finite element model typically includes a plurality of discrete elements that are arranged to simulate a particular structure or environment. Importantly, each element in such a model can be individually pre-programmed to exhibit particular physical characteristics of the structure, or environment, that is being modeled. Of particular importance for the present invention is the structure for the cornea of an eye.
Anatomically, the microstructure for the cornea of an eye includes five identifiable layers of tissue. Proceeding in a posterior direction from the anterior surface to the posterior surface of the cornea, these layers are: 1) Epithelium, 2) Bowman's Capsule, 3) Corneal Stroma, 4) Descemet's membrane, and 5) Endothelium. Of these tissue layers, Bowman's capsule and the corneal stroma are, bio-mechanically, the most important for purposes of reshaping the cornea. Specifically, although it is relatively thin (˜12 μm) Bowman's capsule is approximately five times stronger in tensile strength than the next strongest tissue, the corneal stroma. The corneal stroma, however, comprises approximately eighty percent of the cornea (˜500 μm).
It happens that when modeling the cornea, only Bowman's capsule and the corneal stroma need be considered for most practical applications. In these models, and in line with the anatomical and biomechanical factors mentioned above, the stronger Bowman's capsule requires more finite elements than does the corneal stroma. It also requires higher stress scaling coefficients. As for boundary conditions on the finite element model, it is generally accepted that a sufficient approximation of an actual cornea can be made by pre-programming elements to represent the periphery of Bowman's capsule. Specifically, these peripheral elements can be effectively pre-programmed to represent a fixed attachment of the cornea to the limbus (sclera) of the eye.
In conjunction with recently developed laser surgical protocols, a finite element model gives promise of being able to effectively predict refractive surgery results. Specifically, U.S. Patent Applications for inventions respectively entitled “Method for Intrastromal Refractive Surgery”, and “Computer Control for Bio-mechanical Alteration of the Cornea”, both of which are assigned to the same assignee as the present invention, have addressed surgical protocols for reshaping the cornea. In accordance with disclosure from these applications, the protocols create weaknesses in stromal tissue that result in a redistribution of bio-mechanical stresses and strains in the cornea. With this redistribution, the objective is to then have intraocular pressure from the anterior chamber of the eye force a reshaping of the cornea for the purpose of correcting the refractive power of the cornea.
In light of the above, it is an object of the present invention to provide a finite element model that responds to the LIOB of corneal tissue, to thereby predict a consequent reshaping of the cornea. Yet another object of the present invention is to provide a system and a method for evaluating changes in the bio-mechanical stress-strain distributions within the cornea, in response to predetermined LIOB. Still another object of the present invention is to provide a system and method for simulating and modeling the reshaping of a cornea that is relatively simple to implement, is easy to use and is comparatively cost effective.